Young rats were given either a single s.c. injection (1 mg at 0, 1, 4 or 8 days), or 4 consecutive daily injections (0.2 mg/day between 0 and 3 days; 0.4 mg/day between 4 and 7 days; 0.6 mg/day between 8 and 11 days) of cortisol acetate in order to test the influence of age on the action of corticosteroids on the biochemical maturation of the cerebrum and cerebellum in terms of their DNA, RNA and protein contents. The diminution of the DNA content at 35 days was greater in the cerebellum (-16 to -32%) than in the cerebrum (-9 to -20%); the DNA content of the cerebrum was more affected by treatment at birth, whereas that of the cerebellum was more affected by the delayed treatments. Results were different when expressed in terms of reduction of the normal increase: the gain of DNA decreased more in the cerebrum (-70%) than in the cerebellum (-40%). The most delayed treatment induced a greater effect in both organs. These abnormalities were not always accompanied by a significant decrease of the body weight. The treatments generally led to an increase of the mean cell territory, expressed either in terms of decrease of the DNA concentration, or in terms of increase of the organ weight/DNA ratio. The increase of the RNA/DNA and the protein/DNA ratios constituted an indication of an accelerated cellular maturation.